(1) Field of the Invention
The present invention relates to a method for developing electrostatic latent images. More particularly, the present invention relates to an improvement in the method for developing an electrostatic latent image with a magnetic brush of a magnetic developer, in which good earing of the magnetic brush is maintained.
(2) Decription of the Prior Art
As the developer for use in the magnetic brush development of an electrostatic latent image, there are known two types of developers, that is, a two-component type developer comprising a mixture of an electroscopic powder (toner) having a fixing property and a magnetic carrier and a one-component type magnetic developer comprising particles of a dispersion of a magnetic powder in a binder medium having a fixing property.
In the two-component type developer, it is necessary to charge the toner particles by friction of the toner with the magnetic carrier, and this developer is defective in that a good image cannot be obtained unless the mixing ratio between the magnetic carrier and the toner is kept within a certain range. Furthermore, while the developer is used for a long time, the toner or toner component is deposited and accumulated on the surface of the magnetic carrier (so-called spent toner is formed), and charging of the toner becomes difficult.
The one-component type magnetic developer is advantageous in that troublesome use of a magnetic carrier as in case of the two-component type developer can be avoided, but when the one-component type magnetic developer is used, it often happens that earing of the magnetic brush is considerably changed according to environmental changes such as changes of the temperature and humidity and it is difficult to keep a constant earing quantity of the magnetic brush. In case of the one-component type magnetic developer, it is the developer per se that forms ears of the magnetic brush, and since earing is effected while the developer particles are moved on a sleeve with rotation of the developing sleeve or rotation of a magnet in the sleeve, if the flowability (which also is changed according to the environmental changes) is changed, the earing quantity of the magnetic brush is changed.
Furthermore, in the one-component type magnetic developer, various problems arise because the threshold value of the development is determined according to the magnetic force of attracting the developer to the sleeve and the Coulomb force of attracting the developer to the electrostatic latent image. More specifically, since the magnetic material is embedded in a resin, the magnetic force of attracting the developer to the developing sleeve is weaker than the attractive force of the magnetic carrier, fogging is readily caused at the development. Furthermore, if the flowability of the developer particles is reduced, the frictional charging among the particles becomes insufficient, with the result that the image density is reduced.